Revise CONTRIBUTING.md for branch targeting and AI guidelines

[akarnokd]

Update it to include past 6 months of activity experiences and policies.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.647 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

CONTRIBUTING.md: 4.647

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (32bb3d6) to head (3392a6b).

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8248      +/-   ##
============================================
- Coverage     98.60%   98.59%   -0.02%     
+ Complexity     7158     7157       -1     
============================================
  Files           822      822              
  Lines         50683    50683              
  Branches       6889     6889              
============================================
- Hits          49978    49971       -7     
- Misses          483      485       +2     
- Partials        222      227       +5     

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